Before I get started with a few hasty remarks on today’s events, let me remind you that Lonnie Thompson’s Frontiers in Geophysics lecture will be webcast live on Wednesday at 1815 Pacific time. A link to the webcast can be found here. The lecture is entitled “Abrupt Climate Change and Our Future”. At the same page you’ll find links to Arvidson’s Whipple lecture on Mars exploration, which will be webcast at 14:20. Enjoy! Wish you were here.
Now, let me say at once how inspiring it is to see so much first-rate innovative science arrayed here. There are a lot of geophysicists in the world, and most of them are very, very good. It is especially encouraging to see so much new, young talent in all areas. I spend all too much of my time on RealClimate writing about bad science, it is great to come here and get a reality check.
I spent a fair chunk of my morning at the session at which I myself was giving a talk. This session was “Studying Climate Dynamics with Idealized Atmospheric GCM’s,” and was one of the few sessions devoted to basic fluid dynamical issues having a bearing on how climate works. By “idealized GCM’s” one means things like simplifying the radiation or continental configurations or thermodynamics, but retaining all or most of the fluid dynamics. What this really is about is how to design GCM simulations so as to test hypotheses and develop general understanding. That sort of thing doesn’t usually come from the fanciest and most comprehensive model. This session included such things as the way storm scale (“baroclinic”) eddy feedback helps determine the North Atlantic Oscillation. or how the sharpness of tropical sea surface temperature gradients determine the nature of the Intertropical Convergence Zone (an especially fascinating talk by Tim Dunkerton and colleagues). I myself was presenting a potpourri of some of the dynamical issues in my work on Titan and on the Snowball Earth postglacial hothouse — e.g. the reason that precipitation fails to rise as fast as Clausius-Clapeyron would suggest (which has some commonalities with methane precipitation limitation on Titan, where you only have 2 Watts per square meter to drive all the hydrological cycle). We had a quite good audience, but not the kind of SRO crowd that the Greenland Glacier session had. It can’t escape one’s attention that the amount of interest in and attention paid to basic fluid dynamical issues in climate is a lot less than what is going into some other areas. Someday, the balance will need to be restored. One problem is that fluid dynamics is really hard, and it is even harder to get into the newspaper with it, because it’s hard to explain to people. That makes it have less flash, though it’s none the less important to the understanding of climate. Still, a good time was had by all.
For me, the high point of the day (apart from an impromptu and wide-ranging conversation with Kevin Zahnle on SO2 photolysis, problems that have emerged with some classic Methane Early Earth radiative calculations, the possibilities of N2O as a greenhouse gas on the young Earth, and the general issue of oxygenation of planets) was Mark Serreze’s Nye Lecture on Arctic climate change. The tough part of that was that (as seems to be a law of the Universe) the Gods of Scheduling at AGU scheduled Mark’s talk right on top of David Lea’s Emiliani lecture on tropical climate change during the Last Glacial Maximum. So, one had to decide whether to go Cold or go Warm. I had been struggling with this decision all day, but in the end the decision was made for me since Mark’s talk was closer to the beer and by the time I was done talking to Kevin, there was no time to get over to Moscone West for David’s talk.
Needless to say, Mark spoke to a huge ballroom, and it was packed to the gills. This really is the Year of the Ice. He opened by showing some quotes from his earlier papers. Seven years ago, big changes in the arctic were clearly observable, but he wouldn’t have said with confidence that they were due to anthropogenic global warming (AGW). The problem is that the Arctic is subject to a lot of natural variability, notably the North Atlantic Oscillation (NAO). There was NAO-related warming in the period 1920-1940, so how could we know that the current warming wasn’t part of the same thing?
What changed Mark’s mind was that improvements in climate models made it easier to see the anthropogenic component of climate, as did a few more years of the AGW signal rising above the background variability. This is true not just for global means, but for regional climate. More importantly, there as been a persistent warming and sea ice retreat despite circulation changes (e.g. NAO going neutral) which should be forcing ice the other way. Mark said that the circulations that used to help you don’t help you build sea ice anymore, and the ones that hurt are now hurting more. That latter effect is intimately connected with a general thinning of sea ice,which makes it more vulnerable.
Models are getting good, he pointed out, but reality is exceeding expectations. Since 2005 there has been a 25% decline in Arctic sea ice at the time of minimum, equal to the combined area of Texas and California. What’s more, the drop in 2007 was way below the already steep trend line for 1978-2002. Factors contributing to the unusual 2007 drop include an unusual pattern of atmospheric circulation, with high pressure over the Central Arctic and low pressure over Siberia. This brings a lot of warm air into the Arctic. How does this fit into the longer term pattern? It comes back to the thinning again: the sea ice was so vulnerable to this situation (which has happened before) because it is so thin.
He compared sea ice trends between observations and the IPCC Fourth Assessment Report models. The observed rate of ice loss is much faster than expected. Even before 2007, the downward trend is at least twice the GCM trend, and 2007 is of course way more extreme than even that. The GCM’s say that 40% of the sea ice loss is anthropogenically forced between 1953 and 2007, and if you just count from 1979, it’s more like 50%. But that’s the GCM’s. In reality, given the extreme observed loss, the anthropogenic component may be much greater. It’s the ice thickness again: GCM’s seem to be overestimating ice thickness.
Other big changes are happening. Igor Polyakov’s results show pulses of increasingly warm Atlantic water, seen in moorings at Svinoy and Fram Strait. Also, there are feedbacks associates with Pacific summer water: Loss of sea ice means the atmosphere warms more, and that helps shunt more Pacific summer water farther into the Arctic Ocean, melting yet more sea ice, etc.
Are we at a tipping point, Mark wonders? He pointed to an NCAR simulation with the model CCSM3, which shows a gradual sea ice decline until 2020, but then a kick from an atmospheric fluctuation causes a sudden near-total disappearance of the thinned ice, which was primed to pop. The state of ice thickness observed in 2007 is quite similar to the modelled state in 2020. Mark suggests (my paraphrase) that 2007 is the new 2020.
Is the 2007 drop the first harbinger of a shattering drop like the NCAR model has in 2020? We’re close to the critical ice thickness now, based on IceSat data. Will 2007 be remembered as the tipping point for sea ice? Only time will tell.
Based on a detailed comparison of observed patterns of Arctic warming with GCM patterns including anthropogenic forcing, Mark declared confidently that “Arctic Amplification is Here.” The simulated patterns are spot-on the observed patterns, both with regard to seasonal cycle (warming is greatest in the cold seasons) and vertical structure (warming is most pronounced near the ground). But — the amplitudes observed today are most like the GCM forecasts for around 2020. Not something to send you off to bed with sweet dreams, exactly.
Mark also pointed to some impacts of sea ice loss:
Two interesting questions that emerged in the Q&A session concerned how polar bears survived the Eemian interglacial. It then came out that we don’t know whether the summer sea ice was gone or not during the Eemian. Another question, related to the “tipping point” issue is whether the loss of sea ice is a “bifurcation” where you go from the icy state to an ice-free state discontinuously when the temperature passes a threshold, or whether it’s a continuous transition. Mark feels that it’s probably a bifurcation, based on the effects of sea ice thinning, but it’s basically not really known yet.
That’s all for tonight! Back tomorrow with more great stuff from AGU.
Postscript: Alan Robock, president-elect of the Atmospheric Sciences Section, organized a very fine Chinese banquet for the Section. Entertainment was provided by the “Physics Chanteuse,” whose songs often could use a bit of tweaking for fidelity to basic physics, but were always entertaining. She did a lot of the Hy Zaret repertoire, though unfortunately didn’t seem to be up on Tom Lehrer. A snippet: The Sun is a mass/ of incandescent gas/ A gigantic Nuclear Furnace…